1. Field of Invention
The invention relates to a method of lubricating a mold for continuously casting metal, more particularly steel.
As is known, in the method of continuously casting a metal, the liquid metal is poured into a bottomless mold, usually vibrated, whose peripheral wall is vigorously cooled. As a result of the vigorous cooling, a skin of cast metal quickly solidifies, after which the metal strand is extracted from the mold and travels through withdrawal rolls where solidification continues to completion.
When the metal begins to solidify in the mold, two solid surfaces are juxtaposed, one moving relative to the other. These two surfaces are at high, though different temperatures, and consequently there are serious problems of friction and therefore of lubrication in the contact zone.
2. Prior Art
Conventionally, a continuous-casting mold of the kind in question is lubricated by depositing a powder containing a suitable lubricant on the upper surface of the cast metal. The high temperature in the mold melts the powder, which, owing to the vibration of the mold, penetrates between the cast metal and the inner surface of the mold. It may happen, however, that the lubricant loses some or all of its properties, inter alia its fluidity, before reaching the lower end of the mold. This may occur e.g. because the molten powder entrains particles such as alumina inclusions from the cast metal. These particles, which are usually refractory, raise the melting-point of the powder, which solidifies irregularly so that the mold is no longer lubricated in a hydrodynamic manner.
Also, at the lower end of the mold, the cast metal has a solidified skin which is already of considerable thickness, i.e. the skin is cool compared with the molten metal. As a result, the vigorously-cooled mold wall is at a temperature appreciably below the melting point of the powder. The inner surface of the wall therefore becomes covered with a layer of solidified powder, which disadvantageously affects the film of lubricant and reduces its efficiency.
In the present state of the art, therefore, the lower part of a continuous casting mold is not always lubricated in a satisfactory manner, which may be a serious disadvantage with regard to the surface quality of the cast metal.